Heat transfer from a hot fluid to a cold fluid is an important process in virtually all chemical processing, power generation, and oil & gas facilities. The energy efficiency of these plants depends on effective heat transfer. Improvements in waste heat recovery, recovery of waste products, and recycling of waste streams often involve a heat transfer application in which there is a tendency to scale or foul the heat transfer surface. These applications often require heat transfer with small differences in the temperature between the hot and cold fluids and operation at high temperatures and pressures. Heat transfer equipment which loses heat transfer capacity due to fouling and scaling results in lost economic value in the form of lost heat, production, and the labor required for cleaning the equipment. Such demanding heat transfer applications would benefit from heat transfer equipment which is resistant to fouling and scaling.
Zero Liquid Discharge (ZLD) wastewater treatment systems often use evaporation to recover pure water from waste brines. Waste brines often have salts with inverse solubility. As the brine is heated in the evaporator salts precipitate on the heat transfer surface. Calcium sulfate seeded slurry evaporation can be used in applications in which there is a sufficient amount of calcium and sulfate present. In other applications in which the chemistry is not suitable for seeded slurry operation the evaporators must be cleaned frequently.
Enhanced oil recovery processes, including Steam Assisted Gravity Drainage (SAGD) for oil sands, generate produced waters, or waste streams generated by the reuse of produced water, both which have the characteristics of a high pH, and the inclusion of dissolved silica, suspended particulates, oil and grease, and dissolved organics. These streams are referred to as high silica process affected water (HSPAW).
HSPAW is thus a byproduct of the SAGD process used to recover heavy oil. In a SAGD process, steam is injected into an oil bearing formation to heat and thereby reduce the viscosity of the oil. After the steam condenses, it mixes with the oil, and both the oil and water flow to a collection well and then to a separator wherein the oils are separated from the water. After the water leaves the separator it flows to a polishing deoiler where further oil and solids are removed resulting in production water.
The deoiled HSPAW must be treated to remove scale forming constituents, such as silica and hardness, before the water can be reused, for example in the generation of steam. The traditional method for generation of steam in enhanced oil recovery is to utilize a once through steam generator (OTSG) in which steam is generated from a treated feedwater through tubes heated by gas or oil burners. The OTSG feedwater may have a total dissolved solids concentration as high as 8,000 ppm, and requires a hardness level that is less than or equal to 0.5 ppm (as CaCO3) and a silica concentration that is less than or equal to 50 ppm (as SiO2). This method produces a low quality or wet steam, which is approximately 80% vapor and 20% liquid, at pressures ranging from 250 pounds per square inch gauge (psig) up to 2400 psig. This 80% quality steam is either directly injected into the formation; or in some cases the 80% vapor is separated from the 20% water and then the vapor is injected into the formation. Either a portion or all of the 20% blowdown, which has a concentration of dissolved organics of approximately 8,000 ppm and a concentration of silica of approximately 250 ppm (as SiO2) is disposed as a wastewater, usually through deep well injection.
Heat exchangers are part of the above described steam generation process, however, fouling and scaling of the heat transfer surface in heat exchangers reduces heat transfer efficiency and capacity. Various heat exchanger designs and operating practices have been developed to deal with this fouling and scaling. All of these methods have one or more of the following limitations: pressure and temperature limitations; mechanical cleaning not being practical; fouling/scaling material removal requiring off-line chemical or mechanical cleaning; on-line fouling/scaling devices not being monitored during normal operation; and a high cost.
There are two broad categories of heat exchanger: plate and tubular. Plate heat exchangers are configured as either plate and frame, or spiral. Both these types of plate heat exchanger are limited to pressures in the range of 10 to 20 barg. The plate type designs are not suitable for on-line mechanical cleaning. Tubular heat exchangers, containing a plurality of tubes, are capable of operation at 100 barg or higher. Most of these designs are variations on the shell and tube type. The shell and tube types with tubesheets are suitable for both on-line and off-line mechanical cleaning. In both on-line and off-line cleaning for heat exchangers with more than several hundred square meters of heat transfer surface, there are tens to hundreds of tubes. The length to diameter ratio of heat transfer tubes in typical tubular heat exchangers is in the range of 100 to 700.
For on-line brush cleaning of shell and tube heat exchangers, each tube must have a brush and a housing at both ends of the tube devices for receiving the brush. None of the on-line configurations available provide a practical method for monitoring the status of the on-line cleaning device. For off-line mechanical cleaning, the cleaning process must be repeated for each and every tube. Off-line cleaning of shell and tube heat exchangers with hundreds of tubes often takes 5 to 10 days.
There is a lack of heat exchangers wherein on-line cleaning devices can be monitored, which can operate at pressures between 50 and 100 barg, and which can be economically cleaned offline.